GD4.1

Several geochemical tracers (S, C, O, Xe) underwent irreversible global changes during the Precambrian, and in particular during the Great Oxygenation Event (GOE), between the Archean and Proterozoïc eons [1]. Xenon is of particular interest as it presents a secular isotopic evolution during the Archean that ceased around the time of the GOE. In this regard Xe is somewhat analogous to mass-independent fractionation sulfur (MIF-S) in that it can be used to categorically identify Archean atmospheric components [2]. Xe isotopes in the modern atmosphere are strongly mass-dependent fractionated (MDF-Xe), with a depletion of the light isotopes relative to the heavy ones. There was a continuous Xe isotope evolution from primitive Xe to modern Xe that ceased between 2.6 and 1.8 Ga [2] and this evolution has been attributed to coupled H⁺-Xe⁺ escape to space [3].

The purpose of this project is to document the Xe composition of the paleo-atmosphere trapped in well-dated hydrothermal quartz fluid inclusions with ages covering the Archean-Proterozoic transition to better constraint its link with the GOE.

We have measured an isotopically fractionated Xe composition of 2.0 ± 1.8 ‰ relative to modern atmosphere at 2441 ± 1.6 Ma, in quartz vein from the Seidorechka sedimentary formation (Imandra-Varzuga Greenstone belt, Russia). A slightly younger sample from the Polisarka sedimentary formation (Imandra-Varzuga Greenstone belt, Russia) of 2434 ± 6.6 Ma does not record such fractionation and is indistinguishable from the modern atmospheric composition. A temporal link between the disappearance of the Xe isotopes fractionation and the MIF-S signature at the Archean-Proterozoic transition is clearly established for the Kola Craton. The mass-dependent evolution of Xe isotopes is the witness of a cumulative atmospheric process that may have played an important role in the oxidation of the Earth's surface [3], independently of biogenic O₂ production that started long before the permanent rise of O₂ in the atmosphere [4].

[1] Catling & Zahnle, 2020, Sciences Advances 6, eaax1420. [2] Avice et al., 2018, Geochimica et Cosmochimica Acta 232, 82-100 [3] Zahnle et al., 2019, Geochimica et Cosmochimica Acta 244, 56-85. [4] Lyons et al., 2014, Nature 506, 307-315.

How to cite: Ardoin, L., Broadley, M., Almayrac, M., Avice, G., Byrne, D., Tarantola, A., Lepland, A., Saito, T., Komiya, T., Shibuya, T., and Marty, B.: The end of the atmospheric xenon Archean’s evolution: a study of the Great Oxygenation Event period, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9527, https://doi.org/10.5194/egusphere-egu22-9527, 2022.

Photosynthesis, the metabolic route for conversion of solar to chemical energy, could be present in any planetary system provided with the only three required ingredients: a light source, water, and carbon dioxide.

The ExoPhot project aims to study the relation between photosynthetic systems and exoplanet conditions around different types of stars (i.e. stellar spectral types) by focusing on two aspects: Assessing the photosynthetic fitness of a variety of photopigments (either real or theoretical) as a function of stellar spectral type, star-exoplanet separation, and planet atmosphere composition; and delineating a range of stellar, exoplanet and atmospheric parameters for which photosynthetic activity might be feasible. In order to tackle this goals, this project is studying the evolutionary steps that led to the highly evolved chlorophylls and analogues, and assessing the feasibility or likelihood to trigger photosynthetic activity in an exoplanetary system.

Based on the Darwinian theory of common ancestors, the first (photosynthetic) organism should have had simple oligopeptides, oligonucleotides and alkyl amphiphilic hydrocarbons as primeval membranes. Therefore, it should have had simple pigments. We propose that there could exist geochemical conditions allowing the abiotic formation of a simple pigment which might become sufficiently abundant in the environment of an exoplanet. Besides, we show that the proposed pigment could also be a precursor of the more evolved pigments known today on Earth by proposing, for the first time, an abiotic chemical route leading to tetrapyrroles not involving pyrrole derivatives.

Juan García de la Concepcióna,* Pablo Marcos-Arenala, Luis Cerdánb, Mercedes Burillo-Villalobosc, Nuria Fonseca-Bonillaa,María-Ángeles López-Cayuelad, José A. Caballeroe, and Felipe Gómez Gómeza

aCentro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir km. 4, Torrejón de Ardoz, 28850 Madrid, Spain; bInstituto de Ciencia Molecular (ICMoL), Universidad de Valencia, 46071 Valencia, Spain.;cInstituto Nacional de Técnica Aeroespacial, 28850 Torrejón de Ardoz, Madrid, Spain.; dÁrea de Investigación e Instrumentación Atmosférica,Instituto Nacional de Técnica Aeroespacial, 28850 Torrejón de Ardoz, Madrid, Spain.; eCentro de Astrobiología (CSIC-INTA), ESAC, camino bajo del castillo, 28691 Villanueva de la Cañada, Madrid, Spain

How to cite: García de la Concepción, J. and Marcos-Arenal, P.: ExoPhot: Phot0, a plausible primeval pigment on Earth and rocky exoplanets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13571, https://doi.org/10.5194/egusphere-egu22-13571, 2022.

Biogenicity and taphonomy of the early life fossil records are debated as most of the previous studies focussed mainly on isotopes geochemistry. The non-metamorphosed Paleoproterozoic (~2.1 Ga) sedimentary succession of the Francevillian Basin (Gabon) contains the oldest complex multicellular organisms embedded in black shale facies. Several studies have confirmed the biogenicity of these soft-bodied organisms. Here, we used multi-proxy techniques to show that the preservation of these macro-organisms happened in a close system that limits interaction with their host rocks, which leads to their good preservations. The macro-organisms are present in different shapes and sizes: lobate (L), elongate (E), tubular (T), segmented (S), and circular (C), and are often associated with bacterial mats. Except for the C form, most of the other specimens are pyritized. Sulfur isotopes data confirms that pyritization occurred by bacterial sulfato-reduction during early diagenesis. We compare the clay mineral assemblages between the pyritized specimens and the late-diagenetically formed pure pyritized concretions in the sediments because the early pyritization process could not explain the taphonomic preservation alone. Our clay mineralogical data show that the specimens are dominated mainly by randomly mixed layer Illite-smectite (IS MLMs), illite, and chlorite relative to the host rocks. The abundance of IS MLMs indicates incomplete illitization of smectite, potassium deficiency, and limited mineral reactions in a semi-close local chemical system within the fossils.  In addition, the authigenic chlorites are more iron-rich and show vermicular habitus. By contrast, the pyritized concretions mainly consist of well-crystallized illite and less iron-rich chlorite, while the smectite phases are absent. These results confirmed that the diagenetic reaction is controlled by interaction with an open late diagenetic system. We concluded that taphonomic preservation of the ancient fossil record resulted from the early diagenetic growth of pyrite crystals during bacterial sulfato reduction in the fossils, which creates a semi-closed system that drastically reduced fluid-rock interactions with the host sediments.

How to cite: Ngwal Ghoubou Ikouanga, J. A., Fontaine, C., M. Bankole, O., Laforest, C., Riboulleau, A., Trentesaux, A., Boissard, C., Somogyi, A., Meunier, A., and El Albani, A.: Taphonomy of early life: Role of organic and mineral interactions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-955, https://doi.org/10.5194/egusphere-egu22-955, 2022.

The origins of Eoarchean peridotites found in the Itsaq Gneiss Complex (IGG) of southern West Greenland represent a crucial record of igneous and geodynamic processes on the early Earth. The igneous and geodynamic origins of these rocks have, however, been the subject of controversy, with some researchers arguing that they represent the first known slivers of mantle emplaced by tectonic processes in the crust and others contending that they represent cumulates associated with the local basalt units. The geodynamic context for the formation of these rocks has also been disputed, with some researchers arguing that they formed in a horizontal tectonic setting analogous to a modern subduction zone, while others propose a vertical tectonic origin for all Eoarchean rocks. Here, we provide new insights into the history of these peridotites using multiple sulfur isotope signatures combined with Hf isotope compositions. Anomalously high εΗf values in some IGC peridotites identified in previous studies [1], as well as in metabasalts with boninite-like compositions [2] found in the Isua Supracrustal Belt (ISB) within the IGC, point to contributions from a mantle source already depleted in the Hadean [2]. The multiple sulfur isotope data of the IGC peridotites found south of the ISB reveal small but significant Δ³³S anomalies, consistent with incorporation of surface-derived material of Archean age or older. Furthermore, correlations between sulfur isotope data and major and trace element abundances as well as initial Hf isotope values of IGC peridotites support the hypothesis that high-degree melt depletion occurred under hydrous conditions, followed by variable degrees of melt metasomatism. The involved fluid and melt components precipitated sulfides that incorporated surface-derived sulfur with different depositional origins. We propose that these findings are best explained by a horizontal tectonic regime similar to modern arc settings.

1. van de Löcht, J., et al., Preservation of Eoarchean mantle processes in ∼3.8 Ga peridotite enclaves in the Itsaq Gneiss Complex, southern West Greenland. Geochimica et Cosmochimica Acta, 2020. 280: p. 1-25.

2. Hoffmann, J.E., et al., Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland. Geochimica et Cosmochimica Acta, 2010. 74(24): p. 7236-7260.

How to cite: Lewis, J., Hoffmann, J. E., Schwarzenbach, E. M., Strauss, H., Li, C., Münker, C., and Rosing, M. T.: Sulfur and Hafnium Isotope evidence for Early Horizontal Tectonics in Eoarchean Peridotites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5226, https://doi.org/10.5194/egusphere-egu22-5226, 2022.

The nature of Paleoarchean (>3.2 Ga) crustal accretion continues to be debated, in particular the onset and timing of subduction-like processes. Crust of this age is typically characterised by dome-and-keel geometry that is widely interpreted to be related to “sagduction” or the episodic dripping of denser, mafic volcanics into the mantle around buoyant silicic cratonic nuclei. This occurs during regional scale crust-mantle overturn events.

The exceptional preservation of the East Pilbara Terrane (EPT) has been instrumental in the development of this model and its role in Paleoarchean continental crust formation. The Emu Pool Supersuite (~3324-3290 Ma) represents a phase of voluminous silicic magmatism that has been attributed to overturn and sagduction within the EPT (e.g. Wiemer et al., 2018). However, the widespread occurrence of magmatic-hydrothermal Cu and Mo mineralisation, reported to be linked to this magmatic episode, have received little attention. Comparisons to Phanerozoic porphyry Cu-Mo deposits have been drawn (e.g. Barley & Pickard, 1999), which is intriguing as such porphyry-type deposits have a clear genetic link to arc magmatism and subduction processes as they require hydrous, Cl-rich magmatism (e.g. Tattich et al., 2021).

To date the chronological relationships of the magmatic-hydrothermal deposits to the major dome forming silicic magmatism is poorly constrained. In one deposit, hydrothermal activity is constrained by ¹⁸⁷Re-¹⁸⁷Os geochronology (Stein et al., 2007) to late to post Emu Pool Supersuite magmatism, yet this interpretation is hampered by issues relating to the λ¹⁸⁷Re uncertainty. Furthermore, interpretation of Paleoarchean geodynamics and magmatic evolution generally relies on micro-beam zircon U-Pb geochronological analyses, typically reported at single ²⁰⁷Pb/²⁰⁶Pb date precision at >±10 Myrs (2s), and presents challenges for accurately resolving geological processes and events.

We demonstrate that high-precision CA-ID-TIMS (Chemical Abrasion-Thermal-Ionisation Mass Spectrometry) zircon U-Pb geochronology, utilising ATONA low-noise detectors, can now routinely obtain precision of  ~<±200 kyrs (2s) on ²⁰⁷Pb/²⁰⁶Pb dates of single zircon or fragments at ~3.3 Ga. By combining detailed field relationships, with unprecedented temporal precision, we show that the Mo-Cu hydrothermal mineralisation can be demonstrably linked to their host plutons and formation timescales can even be constrained to ~1 Myrs, comparable to Phanerozoic porphyry deposits. This study identifies that magmatic-hydrothermal systems were not synchronous across the EPT. Instead they occurred over >7 Myrs during the early phase of Emu Pool Supersuite and silicic magmatism within domes.

Whilst the geodynamic trigger for Mo and Cu magmatic-hydrothermal mineralisation at ~3.3 Ga remains enigmatic, we highlight their timing and occurrence should be accommodated within Paleoarchean geodynamic models. Furthermore, the results illustrate the potential of modern high-precision U-Pb geochronology to routinely examine Paleoarchean magmatic records at timescales that closely approximate known plutonic accretion rates within the Phanerozoic.

References

Barley, & Pickard, (1999) Precambrian Research, 96, 41-62

Stein et al., (2007) Geochimica et Cosmochimica Acta, 71

Tattitch et al., (2021) Nature communications, 12, 1-11.

Wiemer et al., (2018) Nature Geoscience, 11, 357-361.

How to cite: Thijssen, A., Tapster, S., and Parkinson, I.: Pinpointing Paleoarchean magmatic-hydrothermal events during the geodynamic and crustal evolution of the East Pilbara Terrane, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7947, https://doi.org/10.5194/egusphere-egu22-7947, 2022.

Singhbhum Craton, eastern India, exposes an array of Paleoarchean granitoids (e.g., TTGs and diorites, transitional TTG, and K-rich granite) ranging in age from ~3.53─3.25 Ga, thus making it a suitable archive for understanding crustal architecture and dynamics during that era. Granitoids cover the core of the craton as a composite dome and are fenced by keels of contemporaneous iron ore bearing greenstone belts from east, west, and south giving rise to a dome-and-keel architecture.  Change in granitoid chemistry and isotope signature over time and space can provide a window into the change of crustal evolution mechanism as well as geodynamics of the crust formation if put into a robust tectonic framework. Most of such earlier studies addressed the secular evolution of granitoid chemistry and isotopic changes as an expression of a shift in tectonic paradigms. This tectonic shift is explained broadly as a response to a progressively cooling earth. However, the timing of the transition (advent of a new tectonic setting) varies globally; hence, each craton needs to be studied separately and without any prior bias.

Spatial variation represented by contour diagrams from the cratonic core show two distinct areas exposing dominantly 3.35–3.25 Ga high-silica, low-magnesiam, high K₂O/Na₂O (K/Na>0.60) granitoids of shallow crustal origin, indicated by their low pressure-sensitive ratios (eg. Eu/Eu*, Sr/Y, Gd/Er, La/Yb). These two areas are surrounded by older intermediate granitoids (>3.35 Ga TTGs). Based on the spatial distribution, it is being suggested that these spatial arrangement of granitoids are related to “partial convective overturn (PCO)” process where the >3.35 Ga TTG basements were subjected to greenstone load while they were soft. As a result some part of the newly formed softer >3.35 Ga TTG crust melted as these overburdens helped in bringing the TTGs to a potential melting depth. The greenstones then sank into the partially molten TTGs along steep-dipping sinistral shear zones by forming synformal keels. The moderate- to- low-pressure TTG-derived partial melts then rose to the shallower level and formed the 3.35–3.25 Ga high-silica, low-Mg# potassic granitoids.

Preserved rock record in the Singhbhum Craton indicates that the granitoid magmatism started at ~3.47 Ga with emplacement of high-silica, low alumina tonalite, characterized by low Sr/Y, (Gd/Er)_N, (La/Yb)_N, Eu/Eu* and Sr. The 3.47 to 3.32 Ga TTG record from the Singhbhum Craton show a progressive increase in Al₂O₃, Sr/Y, (Gd/Er)_N, (La/Yb)_N, Eu/Eu* and Sr and decrease in Na₂O. The increase in the pressure-sensitive ratios reached peak during 3.32 Ga and then started decreasing until ~3.28 Ga followed by another increase during ~3.28 to ~3.25 Ga before ceasing of Paleoarcehan magmatism in the Singhbhum Craton. Such variation in geochemical tracers is explained in terms of episodic crustal thickening by periodic mantle upwelling and associated delamination along with time-progressive changes in bulk chemical composition of the continental crust from mafic to felsic.

How to cite: Mitra, A. and Dey, S.: Time-space evolution of an ancient continent, a window to crustal evolution: Insight from granitoids of Singhbhum Craton, eastern India, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-594, https://doi.org/10.5194/egusphere-egu22-594, 2022.

The Precambrian comprises the vast majority of Earth’s history. Preserved archives contain essential information about the first few billion years for planetary evolution of our planet. Despite covering a large part of the history of our planet, these outcrops are not so abundant due to erosion and frequently occur in disparate areas. In order to relate them and to establish a timeline of geological events in a world lacking biochronology, we rely on accurate radio-isotopic age determinations. These are, however, rather scarce and still leave several hundreds of million years long time intervals undated. In this study, we present U-Pb age determinations from volcanic and sedimentary units of the Paleoproterozoic Transvaal Supergroup, South Africa. The Transvaal Supergroup is an exceptionally well preserved sequence and therefore accounts for a very large amount of geochemical data. Due to its capacity to produce large data sets the preferred technique in U-Pb zircon geochronology for ancient sediments is LA-ICP-MS. It allows the aqcuisition of maximum depositional ages (MDA) in a fast way and at a relatively low cost. However, the large analytical uncertainty preclude the temporal resolution to distinguish between different processes in such old rocks. Moreover, the standard dating procedure rarely includes zircon treatment via chemical abrasion to mitigate common problems such as open system behavior due to radioactive decay damage related Pb loss. In consequence, interpreted ages might be severely disturbed and may yield MDA’s that are tens to hundreds of million years too young. As an alternative, the much more work-intensive CA-ID-TIMS technique allows the obtention of more accurate and more precise ages, preferably using zircon grains that have previously been screened for their LA-ICP-MS U-Pb age.

 Our new combined LA-ICP-MS and CA-ID-TIMS data indicates that the glaciogenic Makganyene Formation has a MDA of ~2.42 Ga. Younger age clusters at around ~2.2 Ga from LA-ICP-MS dating disappear with chemical abrasion and have to be interpreted as artifacts of radiation-damage related Pb loss. These new results have important implications for both environmental evolution during the Neoarchean/Paleoproterozoic, as well as for the regional geology. The Makganyene diamictites are thought to represent the oldest Paleoproterozoic glaciation in South Africa. The data also corroborate the hypothesis that the directly overlying-to-locally-interfingered mafic volcanic Ongeluk Formation is ~200 Ma older than the volcanic rocks ~2250 Ma Hekpoort Formation in the East Transvaal basin. We therefore reject the long-standing correlation between both units, as previously published.

We demonstrate that LA-ICP-MS is not capable to provide a robust and reliable MDA’s in ancient clastic sediments. CA-ID-TIMS analysis provides dates of significantly higher accuracy, because the chemical abrasion is minimizing Pb-loss in the crystal. Therefore, for studies relying on U-Pb zircon geochronology, we encourage the application of CA-ID-TIMS in the youngest populations previously identified with the LA-ICP-MS. This is particularly important for establishing reliable maximum depositional ages in sedimentary rocks.

How to cite: Senger, M. H., Davies, J., Ovtcharova, M., Beukes, N., Gumsley, A., Gaynor, S. P., Ulyanov, A., and Schaltegger, U.: U-Pb zircon geochronology combining both in-situ and bulk-grain techniques in the Transvaal Supergroup, South Africa., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1666, https://doi.org/10.5194/egusphere-egu22-1666, 2022.

The tectonic processes responsible for the formation of early Earth felsic crust (predominantly composed of tonalite-trondhjemite-granodiorite, or TTGs) inform the global regime of mantle convection that operated at this time. Many models have been proposed to explain the formation of Archean TTGs, including melting of downgoing crust in hot subduction zone settings, or melting of crust that is buried by lava flows and founders into the mantle. Formation in a subduction zone setting would imply at least some form of mobile-lid tectonics on the early Earth, while TTG formation via crustal burial and foundering does not require subduction or plate tectonics, and can thus occur in a stagnant-lid regime.  

Regardless of tectonic setting, TTGs can only form if hydrated basaltic protocrust melts before it experiences metamorphic dehydration. Previous work has argued that this constraint may preclude a subduction origin to TTGs. Regional scale numerical models have found that slabs sink quickly and steeply through the mantle at Archean mantle temperatures, such that they dehydrate before melting. However, these models do not consider evolution of grainsize in the mantle interior and in plate boundaries. Using numerical models of mantle convection with grain damage, a mechanism for generating mobile-lid convection via grain size reduction, I show that a sluggish, drip-like style of subduction emerges at early Earth conditions. This subduction style is a result of plate boundaries becoming effectively stronger with increasing mantle temperature, and leads to significant slab heating at shallow depths.

To test whether TTGs can form from this style of sluggish subduction, I use scaling laws developed from numerical models combined with a simple model of the evolution of the vertical temperature profile through a slab. Results show that the slower sinking speed of slabs caused by grain size evolution in plate boundaries allows for crustal melting for a much wider range of mantle temperatures and subducting plate thicknesses than if the effects of grain size evolution were ignored. Overriding plate thickness is also important, with thin overriding plates favored for TTG formation. These results have important implications for the settings where subduction could generate Archean TTGs, and for potential episodicity in TTG formation resulting from both short- and long-term episodicity in subduction.  

How to cite: Foley, B.: Generation of Archean TTGs by slab melting during sluggish, drip-like subduction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10873, https://doi.org/10.5194/egusphere-egu22-10873, 2022.